1. Field of the Invention
This invention relates to a tracking control apparatus for a recording and/or reproducing apparatus on or from an optical disk, and may be advantageously applied inter alia to tracking control by a differential push-pull method.
2. Description of the Prior Art
As a tracking servo control method in a recording and/or reproducing apparatus on or from an optical disk, there has been proposed a tracking servo employing a differential push-pull (DPP) method.
The tracking servo employing this DPP method is an improvement over a push-pull tracking error detection method employing a single laser beam as disclosed for example in the Japanese Patent Publication No. 32735/1986. That is, according to the tracking servo based on the DPP method, the laser light emitted from a laser light source, such as a semiconductor laser diode, is separated by the grating method into three laser beams, i.e. the O'th order beam and .+-. first order diffracted beams. Of these three laser beams, the O'th order beam is irradiated on a recording track TR of an optical disk 1 as a laser spot for reproduction SP.sub.PB, whereas the two .+-.first order diffraction beams are irradiated as two auxiliary laser spots SP.sub.S1 and SP.sub.S2 so that these two beams are offset ahead and back of the laser spot for reproduction SP.sub.PB in the track pitch direction with respect to the recording track TR (FIG. 1).
In this manner, the reflections from the three laser spots SP.sub.PB, SP.sub.S1 and SP.sub.S2, derived from the three laser beams, are obtained. The reflection from the laser spot for reproduction SP.sub.PB is received by a four segment photodetector 2, while the reflections from the two auxiliary laser spots SP.sub.S1, SP.sub.S2 are received by two two-segment photodetectors 3, 4, respectively, each having its division line parallel to the recording track TR of the optical disk 1. On light reception outputs S.sub.A, S.sub.B, S.sub.C, S.sub.D, S.sub.E, S.sub.F, S.sub.G and S.sub.H, produced by light receiving segments 2A, 2B, 2C and 2D making up the four-segment photodetector 2 and light receiving segments 3E, 3F, 4G and 4H of the two segment photodetectors 3, 4, respectively, an arithmetic operation is performed in accordance with the following formula (1) EQU S.sub.TE =(S.sub.A +S.sub.D)-(S.sub.B +S.sub.C) -k.sub.1 (S.sub.E -S.sub.F)+k.sub.2 (S.sub.G -S.sub.H) . . . (1)
wherein k.sub.1 and k.sub.2 are constants.
Meanwhile, with the above described recording and/or reproducing apparatus on or from an optical disk, there has been proposed an optical head in which an optical system exclusive of the optical disk is stationarily arranged in the main body of the apparatus and the objective lens is placed on an actuator and driven thereby to be shifted to a desired recording track on the optical disk to enable high speed accessing. This desired recording track is referred to hereinafter as the access position.
With the above described optical head, supposing that, if an optical axis 6L of an objective lens 6 is positioned with an offset from the O'th order light L.sub.O, among the three laser beams LB.sub.O, LB.sub.+1 and LB.sub.-1 of the O'th and .+-. first order diffraction lights emanating from a stationary optical unit 5, the coupling efficiency of the .+-.first order refraction lights LB.sub.+1 and LB.sub.-1 with respect to the object lens 6 fluctuates as a function of changes in the access positions P.sub.nO, P.sub.n1 of the objective lens 6. The result is that a deviation is brought about in the equilibrium of the sum signal PP2 of second and third push-pull outputs PP21, PP22, which is produced from the two-segment photodetectors 3, 4 and which is represented by the formula EQU PP2=(S.sub.E -S.sub.F)+(S.sub.G -S.sub.H) . . . (2)
with respect to the first push-pull output PP1 which is produced by the four-segment photodetector 2 and which is represented by the formula EQU PP2=(S.sub.A -S.sub.D)+(S.sub.B -S.sub.C) . . . (3)
Thus an inconvenience is caused that the offset produced when the objective lens 6 is off-center with respect to the recording track TR cannot be cancelled.
In addition, since the coupling equilibrium of the .+-.first order diffracted light beams LB.sub.+1, LB.sup.+1 with respect to the objective lens 6 is changed as a function of the access positions of the objective lens 6, an offset is produced in the second and third push-pull outputs PP21 and PP22 from the two two-segment photodetectors 3, 4, so as to be superimposed on the detected tracking error signal S.sub.TE.